Implant free quantum well transistors are already known to the person skilled in the art. Geert Hellings et al. in “Implant-Free SiGe Quantum Well pFET: A novel, highly scalable and low thermal budget device, featuring raised source/drain and high-mobility channel,” IEEE International Electron Devices Meeting—IEDM, pp. 241-244, 2010, European patent application EP 2 120 266 A1 and Geert Hellings et al, in “The implant-free quantum well field-effect transistor: Harnessing the power of heterostructures” in the proceedings of ICSI 2011 conference for example describe such an implant free quantum well (IFQW) transistor.
The transistor comprises a substrate. A multi-layer quantum well region forming the quantum well of the transistor overlies the substrate. Further a gate region forming the transistor gate, shortly the gate, overlies a first portion of the quantum well region while a source region forming the transistor source, shortly the source, overlies a second portion different from and adjacent to the first portion of the quantum well region and a drain region forming the transistor drain, shortly the drain, overlies a third portion different from and adjacent to the first portion of the quantum well region. The source region and the drain region are positioned on opposite sides of the gate region. The substrate comprises a doped region for controlling the mobile charge carrier concentration for establishing an electrical connection between the source and the drain depending on the gate voltage applied to the gate. The doped region extends over the entire substrate at a side of the quantum well region opposing the source region, the drain region and the gate region.
According to a first configuration of such IFQW transistor the doped region, i.e. the substrate, has a low uniformly distributed doping, i.e. a uniform doping having a concentration of 1×1017 particles per cm3. Although such transistors have been found to show a low source/drain capacitance, such transistors have the disadvantage that they have a limited short channel control, a small body factor, and that the VT may be off-target.
According to a second configuration of such an IFQW transistor, the doped region, i.e. the substrate, has a high uniformly distributed doping, i.e. a uniform doping having a concentration of 1×1018 particles per cm3. Although such transistors have been found to show a high source/drain capacitance they show an improved short channel control, a higher body factor and a shifted VT. Related information can be found in Mitard et al in, “1 mA/um-ION strained SiGe45%-IFQW pFETs with raised and embedded S/D” p 134-135 Digest of Technicap papers, Symposium on VLSI circuits (VLSIC) Kyoto 2011.